And it isn't just the gangs who are shooting bystanders. The police in both Rio and Sao Paulo are widely known to shoot or beat people first and ask questions later. Amnesty International filed this report when the current surge in violence began.

Brazilian urban slums are known as farvelas. And while the gang violence in Brazil's two largest cities is not common to other urban slums around the world, they still share the same fundamental traits.

Mark Davis is a Los Angeles journalist and an urban historian and theorist. I highly recommend his book, Planet of Slums, to learn more about what slums are becoming in the 21st century. You thought the late 20th century was bad for the world's urban poor. We not only have extreme poverty and suffering in densely-populated areas, but increasingly, we have military or military-style operations within these areas, which can kill scores of people. In the last few years, Brazil has deployed assault vehicles to engage in firefights with gangs in public streets. These operations have killed criminals and innocents alike.

And 21st century warefare is increasingly urban. The Battle of Mogadishu ("Black Hawk Down") was the US's first 21st century military operation (if you accept the Soviet collapse of August 1991 to be the start of the 21st century as I do). In 2003, Israel experimented with rifles fitted with muzzle video cameras so troops can shoot around corners without exposing themselves in dense urban areas (Gaza). And the US military is designing tiny flying vehicles that can deliver small bombs to living quarters in dense neighborhoods that feature open windows and doors (the slums and shantytowns we see in the third world).